The spotted owl is once again being threatened by human activities, but this time the offending industry isn’t logging – it’s cannabis.

Spotted owls, which remain threatened federally and in California, are being exposed to high levels of rat poison in the forests of northwest California, according to a new study by researchers from UC Davis and the California Academy of Sciences.

The most likely sources of the rat poisons are nearby cannabis farms on private lands in the forests of Humboldt, Mendocino and Del Norte counties, scientists say. But researchers worry that wildlife soon could be at risk throughout California with the legalization of recreational marijuana fueling the spread of pot farms across the Golden State.

Rats, gophers and other rodents can wreak havoc on cannabis, so some growers use poisons to control the pests. The owls become poisoned by feeding on rodents that have eaten the bait, which can take days to kill its intended quarry.

“I really don’t think that most cannabis users know about or want to have this negative effect on the environment,” said Jack Dumbacher of the California Academy of Sciences, who contributed to the study.

The results of the study were striking: 7 of 10 spotted owls found dead in the forest had rat poison in their systems. Poison also was found in 40 percent of the 84 barred owls that researchers collected between 2009-13.

What’s less clear is how the new world of regulated cannabis will affect the numbers of grow operations propagating these poisons.

The study’s lead author, Mourad Gabriel of UC Davis, worries that the impact on wildlife could be devastating.

Rat poisons percolate through the food chain and can kill any animal with rodents on the menu – mountain lions, bobcats, coyotes, other birds of prey, and a host of other predators native to California.

Gabriel’s study suggests the use of rat poison may be widespread, but growers like Pat Malo of Santa Cruz say there are ways to farm cannabis without it.

“Historically these poisons have been used only by unethical growers, and those aren’t the kinds of growers we have here in Santa Cruz,” said Malo. “It’s hard to stop people from doing these things, but regulations are the first step.”

The promise of increased enforcement and regulation that accompany legalization may take years to result in a meaningful crackdown on illegal growers — time that threatened owls may not have.

The spotted owl joined the ranks of imperiled animals on the federal endangered species list in 1990. The old-growth forests they called home in Washington, Oregon and California are estimated to have been reduced by more than 60 percent in the last 190 years, according to the U.S. Fish and Wildlife Service. In 1994 the timber industry came to view the spotted owls as an enemy, as measures designed to protect the owl’s habitat closed swaths of forest to logging.

A mixture of habitat loss and competition from the larger, more aggressive barred owl, a species formerly relegated to the eastern U.S., has fueled their continued decline. Only 560 pairs remain in California, according to the conservation organization Defenders of Wildlife.

Gabriel has known for years the environmental destruction that so-called “trespass growers” — typically members of a drug cartel — have inflicted on California’s public lands.

In 2012, his work connected the deaths of fishers, one of the larger members of the weasel family, and other wildlife to indiscriminate rodenticide use by cartel growers in the remote Sierra Nevada.

But what’s new about his Gabriel’s study, published this month in the journal Avian Conservation and Ecology, is that it points to unlicensed commercial growers on private property as the source of the rat poison. “We didn’t expect to get this result with no cartel sponsored ‘trespass grows’ in the area,” said Gabriel.

Both types of illegal growers are more likely than their newly legal counterparts to use rat poisons, according to Stella McMillin, an environmental scientist at the California Department of Fish and Wildlife.

But cartel-connected “trespass growers” are by far the worst offenders. The California Growers Association, a statewide cannabis advocacy group, sharply distinguishes its members from the cartel growers. “I visited a large cartel grow on public lands. There was a pile of something like 40 carcasses: rats, raccoons, bear, deer. I’ve never been more heartbroken,” said Hezekiah Allen, the group’s executive director.

There is hope among both cannabis advocates and regulators that legalization will reduce environmental impacts by bringing growers into compliance with state and local regulations.

But the new rules can only help if growers are willing to be regulated. Of Humboldt County’s estimated 15,000 grow operations, only 2,000 have applied for licenses, according to a recent environmental impact report. This leaves thousands of growers in the shadows.

While the enforcement crackdown may catch them eventually, there’s no telling how long that will take – or what damage they may do in the meantime.

It may take pressure from consumers to stop growers from using deadly rat poisons and turn to eco-friendly businesses in the cannabis marketplace, said Dumbacher. “Maybe legalization will allow consumers to demand owl-safe marijuana the same way they demanded dolphin-safe tuna.”

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